[18.01] Hazards to Planet Formation

The Orion Nebula provides a remarkable window on the first
few million years in the lives of typical young stars and
planetary systems. HST has demonstrated that most young
stars in the Nebula are surrounded by circumstellar disks
(the so-called `proplyds'). While these observations show
that planet forming environments may be common, they also
demonstrate that Orion's disks are being destroyed by
intense UV radiation fields. `Gravel' sufficiently large to
resist photo-erosion (meter scale solids or ices) may
lock-up sufficient material to eventually build rocky
planets. Indeed, there is evidence for large solids in some
proplyds. But, the hydrogen and helium needed for the
formation of giant planets will be removed.

To form in Orion-like environments, giant planets must be
assembled promptly prior to UV exposure. Even rocky planets
may not form if the photoionized disk corona causes
surviving large particles in the disk to spiral into the
central star. Thus, nearby massive stars pose severe hazards
to planet formation.

Star counts indicate that most stars form in Orion-like
environments. Only about 10% of young stars are born in
shielded environments such as the Taurus or L1641 clouds
where disks may escape photo-erosion. In dark clouds, the
majority of stars (> 80%) form in non-hierarchal multiple
star systems where close encounters with sibling stars can
destroy disks and eject young planets. Thus, most stars may
never develop planetary systems. These considerations
indicate that extra-Solar planets may be rare, contrary to
the popular view. These conclusions are consistent with the
recent discoveries of extra-Solar planets around a few
percent of single stars.